The ting laser gyroscope is a well known device used for measuring inertial rotation. Primarily, the ring laser gyroscope (RLG) is used in navigation systems for aircraft, missiles, etc. In summary, the RLG utilizes two counterpropagating light beams which resonate in a planar closed loop path. Rotation about an axis normal to the plane containing the two counterpropagating light beams causes a difference in the effective pathlength of these light beams. Change in the effective pathlength results in a separation of the resonant frequencies of the two counterpropagating light beams. This frequency shift is a direct indication of the rue of rotation to which the RLG is subjected. Ring laser gyroscopes are more specifically described in U.S. Pat. Nos. 3,373,650 to Killpatrick and 3,390,606 to Podgorski.
As will be recognized by those skilled in the art, the ring laser gyroscope is a very complex device made up of a number of different elements and control systems, all working in conjunction with one another. As a result of the complexity of the ring laser gyro, the cost of production can be somewhat substantial. Efforts are continuously undertaken to reduce this production cost. One method of reducing production costs is to attempt to identify performance characteristics of the ring laser gyro at a point early in its production. By predicting performance characteristics with some degrees of accuracy, units with poor performance characteristics can be removed from the production line, thus eliminating the cost and time required to further process these units. When a unit having bad performance characteristics is identified early during the production process, efforts can be made to improve the performance of this gyro or the unit can be scrapped, thus avoiding further value added steps (extensive testing and processing). Furthermore, early identification of performance characteristics can identify potential problems in the production line and efforts can be made a an early point in time to correct these problems.
One performance characteristic which is crucial to the ring laser gyroscope is the angular random walk (ARW). Angular random walk identifies the stability of the ring laser gyroscope. When incorporated into a navigation system it is important that the ring laser gyroscope only produce signals which are indicative of actual rotation. If the gyroscope is unstable, the possibility of producing erroneous rotation signals is created.
Previously a relationship between a gyro's angular random walk and the gyro's lockin was identified. Due to this relationship, predictions of the angular random walk of a gyroscope were made based on a traditional lockin measurement.
Lockin is a phenomena which is well known in the art wherein the two counterpropagating light beams resonate together or "lockin" at low rotation rates. This lockin characteristic affects many characteristics of the ring laser gyroscope. One specific effect is the non-linearity of the gyro's scale factor of the ring laser gyroscope at very low rates.
While the traditional measurement of lockin did provide a method for predicting angular random walk, this method was not the most accurate. A correlation between the traditional lockin measurement and ARW did exist; however, this was not a strong correlation. Consequently, using the traditional lockin measurement to predict ARW occasionally resulted in the further processing of ring laser gyroscopes which did not perform to the necessary specifications. Referring now to FIG. 1, there is shown a graph which plots the lockin measurement versus subsequent calibrated ARW measurements. The data points are plotted as dots 12 whereas a best fit line 14 is also plotted. As can be seen from FIG. 1 the correlation between the data points and the best fit line is somewhat weak.
Lastly, the previous test used to determine lockin was time consuming and somewhat inaccurate. To test for lockin, the RLG was placed on a rate table and the optical signals within the block were monitored. The table was rotated at a rate known to be above the RLG's lockin rate. Next, the rate at which the table is rotated is decreased to the point at which the two optical signals no longer maintain their independence. This test required close monitoring by a technician which is tedious, costly and time consuming.